The low-temperature X-ray structure, Raman and inelastic neutron scattering vibrational spectroscopic investigation of the non-centrosymmetric amino acid salt glycine lithium sulfate

The structure of the amino acid salt glycine lithium sulfate (GLS) is determined by X-ray diffraction at 90 K and reveals no significant deviations from the previously reported room temperature structure. The vibrational spectrum of GLS is measured at 78 and 298 K by Raman spectroscopy and at 25 K by incoherent inelastic neutron scattering (INS) spectroscopy. There is no evidence of a phase transition in the Raman spectra between 78 and 298 K. Solid-state density functional theory (DFT) is used to simulate the INS spectrum of GLS and to perform a complete normal mode analysis. Discrepancy between simulation and experiment, namely the anharmonic torsional motion of the -NH₃⁺ functional group at approximately 370 cm^-1, is discussed in detail.